1.Field of the Invention
The present invention relates to a selective calling receiver which is capable of selectively receiving calling radio signals and, more specifically, to a wristwatch-type selective calling receiver attached to a pair of bands worn about the wrist of a user, each band having an antenna contained therein.
2. Prior Art
In a wristwatch-type selective calling receiver, since an antenna circuit is incorporated in the bands, the antenna length, or the inductance of the antenna circuit, varies with the wrist thickness of a user, resulting in a variation of the tuning frequency. Since the radio frequency used in a selective calling radio system is fixed, the above type of selective calling receiver can be used only by users whose wrists are in a certain limited diameter range.
A selective calling receiver for solving the above problem is disclosed in Japanese Patent Application Unexamined Publication No. Sho 62-262529. This selective calling receiver 20 has a means for recognizing an antenna length and a control circuit for selecting an optimum one from tuning capacitors based on the recognized antenna length by operating switching devices. The antenna length is recognized by means of an electrical contact provided by a clasp connecting the two bands or by inputting a location of a band connection point. In this manner, the antenna circuit can be tuned to the radio frequency irrespective of the user's wrist thickness.
However, in the conventional selective calling receiver above-mentioned, the control circuit is necessary for identifying the antenna length and operating the switching devices, resulting in complicated circuit configuration and increased cost of manufacturing. Further, because of the necessity of the control circuit and switching devices, the conventional calling receiver consumes excessive power undesirable for a portable device.